Three possible reasons:
1. The streptomycin sensitive cells mutated to resistance.
2, The streptomycin sensitive cells were transformed to streptomycin resistance by DNA- mediated transformation
3. The streptomycin sensitive cells were transduced to streptomycin resistance by transduction
To distinguish between these possibilities, the number of streptomycin resistant cells arising from streptomycin sensitive should be compared under the following conditions:
1. How many resistant cells arise when the sensitive cells are plated on streptomycin containing medium in the absence of resistant cells. The possibility of reversion
2. Is the number of resistant cells reduced if DN’ase is added to both sides of the filter,
3. Irradiate the resistant cell population and look for clearing of the culture to indicate lysis and suggest phage induction.